Various high-frequency filters are used in communications devices. Compactness and improved electrical characteristics are required of filters with prevalent use of cellular or portable telephones and like mobile communications devices. The conventional filters for use at high frequencies include dielectric filters comprising coaxial dielectric resonators as disclosed, for example, in JP-A-283904/1994. The dielectric filter is of the surface mount type which is mountable directly on the surface of a circuit board so as to be compacted.
FIG. 12 is an exploded perspective view showing the dielectric filter of the surface mount type. The filter comprises a plurality of (two in the illustration) coaxial dielectric resonators 100 arranged side by side in contact with each other on a base 7.
FIG. 13 is a perspective view showing the coaxial dielectric resonator 100. The resonator 100 comprises a dielectric body 2 made of a dielectric material and formed with a through bore 22 extending through opposite end faces 20, 21 of the body 2. An outer conductor layer 30, an inner conductor layer 31 and an end conductor layer 32 are provided respectively on the outer peripheral surface of the dielectric body 2, the inner surface of the body defining the bore 22 and one of the end faces, 21, of the body 2. As shown in FIGS. 12 and 13, the resonator 100 has a bottom surface 23 formed by partly removing the outer conductor layer 30. The adjoining surfaces of the adjacent resonators 100, 100 are locally removed to form interstage coupling windows 10, 10. In the case of the surface mount type, the resonator 100 is generally in the form of a rectangular parallelepiped so as to be mountable on the base 7 with ease.
As shown in FIG. 12, the base 7 has strip lines 81, 81 for external connection and a grounding pattern 80 which are formed on the upper surface of a dielectric base plate 70. Each of the strip lines 81 on the upper surface of the base plate 70 extends from an inward region of the plate surface to be in contact with the bottom surface 23 of the resonator 100 to the region of the plate surface to be in contact with the exposed end face 20 of the resonator 100. The grounding pattern 80 is formed on the upper surface of the base plate 70 except inward regions of the plate surface to be contact with the bottom surfaces 23, 23 of the resonators 100, 100 and the regions thereof around the strip lines 81, 81.
The resonators 100, 100 and the base 7 thus constructed are so arranged that the resonators 100, 100 are adjacent to each other and positioned in place on the base 7. In this arrangement, the resonators 100, 100 are electromagnetically coupled to each other through the interstage coupling windows 10, 10, and the strip lines 81, 81 are capacitance-coupled respectively to the inner conductor layers 31, 31 of the resonators 100, 100, whereby a band-pass filter is provided.
While the filter of FIG. 12 comprises two coaxial dielectric resonators 100, 100, at least one coaxial dielectric resonator formed with interstage coupling windows 10, 10 in opposite side surfaces thereof can be disposed between the two resonators 100, 100 adjacent thereto. Generally, the greater the number of resonators joined, the greater is the attenuation of output electric power of signals at the boundary between the pass band and the outside thereof.
To ensure matching between the adjoining resonators 100, 100 of the surface mount dielectric filter thus constructed, the exposed end face 20 of each resonator 100 is adjusted by trimming to obtain a complete product. The completed filter is mounted on a circuit board, with the rear surface of the base in contact with the surface of the circuit board.
To fulfill the requirement that the surface mount dielectric filter be compacted, it is desired to mount the coaxial resonators only on the circuit board with the base 7 omitted.
In this case there arises a need to bond the adjacent resonators to each other. However, coaxial dielectric resonators for use with high-frequency signals have a very small axial length of several millimeters and are therefore difficult to bond as positioned properly.
Furthermore, the coaxial dielectric resonators serving respectively as an input stage and an output stage must be provided with external connection electrodes as separated from the outer conductor layers of the resonators, in place of the strip lines 81, 81 on the base 7. Generally these electrodes are formed near the exposed end faces of the respective resonators so as to be coupled strongly with their inner conductor layers. The resonators serving as the input stage and the output stage are then substantially shortened in resonance wavelength and become higher in resonance frequency. In the case of the dielectric filter comprising at least three adjacent coaxial dielectric resonators of the same size, the input-stage and output-stage resonators then have a resonance frequency higher than, and failing to match, that of the intermediate-stage resonator, hence impaired filter characteristics.
An approximate match in resonance frequency can be obtained by making the intermediate-stage resonator smaller than the other resonators in axial length, whereas the different axial lengths of the resonators result in difficulty in bonding the adjacent resonators in position.